Let's read how a variable's value flows through a small script.
const name = "Ali"; stores the text "Ali" under the name name, permanently this variable will never be reassigned in this script, which is exactly why const fits here.
let age = 16; stores 16 under age, but this time using let, because the very next line, age = 17;, immediately reassigns it. Using const here would cause an error the moment that reassignment ran.
The line document.body.innerHTML = `<p>${name} is ${age} years old.</p>`; uses a template literal (backticks, plus ${...}) to insert the current values of name and age directly into a string you'll cover template literals properly in the next topic.
Run this and check: the page shows "Ali is 17 years old," not 16 because by the time that final line runs, age has already been reassigned. Reading code top to bottom, tracking what each variable currently holds at each line, is a core skill you'll use constantly.